Chin Augmentation With Implant

Daniel A. Cuzzone

Barry M. Zide

DEFINITION

Chin augmentation encompasses genioplasty with the use of mandibular osteotomies as well as placement of implants in the form of grafts or alloplastic materials. Herein we do not discuss osteotomies.
The chin is a defining feature of the lower one-third of the face that connotes a sense of strength to the face.
Correction of microgenia is desired in about 20% to 25% of patients undergoing rhinoplasty, with the goal of balanced facial symmetry.
Implant-based augmentation of the chin may be performed with a variety of implant-based materials, each with its own set of benefits and perceived deficiencies (TABLE 1).
Osteotomies to advance the lower border of the symphysis may accomplish the same thing, but the risks and benefits of each are not described herein.

ANATOMY

The chin, as an anatomical subunit, is composed of the sublabial area and the pad.
The mental nerve is the terminal branch of the inferior alveolar nerve. It is a branch of the third division of the trigeminal nerve and is responsible for sensation of the lower lip, portions of the chin pad, and menton (FIG 1A, blue area) except for a small area that is supplied by a sensory branch off the nerve to the mylohyoid (see FIG 1A, pink area; FIG 1B).
The location of the mental foramen can be variable (FIG 2).
Anatomical studies have demonstrated that the mental foramen is located inferior to the second mandibular premolar in about 50% of adults, between the first and second premolar in 25%, and posterior to the second premolar in 25% of the remaining population.

It is located about halfway between the alveolar ridge and the inferior border of the mandible (8 to 10 mm from the inferior border) and is between 2 and 3 cm lateral to the midline.

Table 1 Implant Subtypes and Characteristics

Material

Trade Name

Tissue Interface

Pros

Cons

Complications

Polydimethylsiloxane

Silastic (silicone rubber)

Fibrous capsule

Easily carved, easily placed and removed

Bone resorption (if too high), seroma, exposure

Malposition, extrusion, infection (may be salvaged)

High-density porous polyethylene

MEDPOR

Su-Por

Limited tissue ingrowth

Versatile

Difficult to remove; requires hardware

Malposition, extrusion, infection (not salvageable)

It may be lower in vertically short height mandibles.
The mentalis muscles are paired mimetic muscles that elevate and compress the chin against the anterior mandible and indirectly raise the lower lip (FIG 3).

PATHOGENESIS

Congenital and acquired factors may contribute the most to a hypoplastic mentum.
- Aging
Dentures that cause trauma to the mentalis origin may lead to soft tissue ptosis and poor projection.

PATIENT HISTORY AND PHYSICAL FINDINGS

A pertinent history and physical exam are instrumental in optimal operative outcomes and should include an appreciation for the patient’s aesthetic goals, history of prior facial surgery, or orthodontic/orthognathic treatment, and a comprehensive facial examination.
Point by point assessment of the lower third of the face¹^,²^,³
- Lip eversion and position
- Occlusion
- Static chin pad thickness (normal 9 to 14 mm)
- Labiomental fold depth and height
- Dynamic chin pad motion while smiling (the pad may be effaced and in some cases may descend)
- Symphyseal narrowness
Assessment of lip position should be noted.
- Normal interlabial gap is up to 3.5 mm.
- Normal lower incisal show at rest is 2 to 3 mm.
- Normal upper incisor show is up to 3.5 mm in males and up to 5.5 in females.
Lower facial height should be noted. Sagittal implant projection will give the face a longer appearance; conversely, if the chin is deficient sagittally but long, an implant will not work well.

Symmetry of the chin should be noted as well as skin irregularities.⁴

FIG 1 • A. Sensory distribution of the mental nerve seen in blue and the contribution of the sensory branch of the never to the mylohyoid in pink. B. Course of the sensory branch off the nerve to the mylohyoid (denoted by the dotted white circle).

Dimpling may be seen after previous surgery, injury, or facial lesion such as Bell palsy.
Assess in repose and during animation both from the frontal and profile view.

When the chin symphysis is narrow, bony advancement may be inadvisable vs prosthetic augmentation.
Dental occlusion
- Prominent teeth may evert the lower lip, producing a more acute labiomental fold. A vertically high implant may worsen this. Also, a skeletally deep bite may seem to shorten the face with the occlusion, allowing the upper teeth to push the lower lip forward.

IMAGING

High-definition preoperative photography is important for photo documentation and for perioperative planning.
Views that should be obtained include frontal and lateral, with and without smile. The smile view shows how the chin pad moves dynamically.
Radiographic evaluation is often reserved for secondary surgical procedures, with short mandibles where the mental foramina may be low, or to evaluate prior hardware.